Geoscience Reference
In-Depth Information
2kmofthe Moho. The uppermost 250 m of the chamber has a very low P-wave
velocity of 3.0 km s
−
1
over a 1.25-km-wide zone, implying the presence of inter-
connected melt. Beneath this thin and narrow lens, the P-wave velocity anomaly
is
0.2 km s
−
1
,which is consistent with a 1% melt fraction. The main difference
between this body and that imaged beneath the Reykjanes Ridge is in its lateral
extent - this is a much narrower feature with smaller velocity anomalies.
−
Fast-spreading ridges
In contrast to the evidence from the slow-spreading ridges, seismic-refraction
and -reflection experiments shot on the East Pacific Rise have given good images
of crustal magma chambers on this fast-spreading ridge. At 9
◦
N, where the half-
spreading rate is 6 cm yr
−
1
,azone with a velocity of about 5 km s
−
1
has been
imaged at the ridge axis by seismic-refraction experiments using ocean-bottom
seismometers (Fig. 9.22(c)). This low-velocity zone was not present in the models
for 2.9- and 5.0-Ma-old crust. Along the ridge axis and at age 2.9 Ma, the mantle
arrivals indicate low P-wave velocities of 7.6-7.7 km s
−
1
.However,byage 5 Ma,
the mantle P-wave velocity is 8 km s
−
1
, and a more typical oceanic crustal layering
appears to have developed. This model was determined from an analysis of the
travel-time and distance data, as well as from waveform and amplitude studies
using synthetic seismograms. Results of subsequent seismic experiments have
confirmed the presence of an axial low-velocity anomaly (maximum
2kms
−
1
)
−
∼
4kminwidth, located between 1 and 3 km beneath the seabed.
Results of another refraction experiment on the East Pacific Rise (at 11
◦
20
N)
using three-component ocean-bottom seismographs indicated that crustal P- and
S-waves both crossed the ridge crest with high amplitudes and no apparent time
delay. If magma is present within the crust at this location, these results indicate
that it must be confined to narrow dykes or isolated bodies less than about 1 km
in vertical extent.
Evidence for the existence of a narrow magma chamber beneath the East
Pacific Rise axis at 21
◦
Nwas provided by the differing attenuation of P- and
S-waves (Fig. 9.24). The P-waves were well transmitted to a seismometer located
on the ridge axis, but the S-waves were severely attenuated. In contrast, both P-
and S-waves were well transmitted to a seismometer located 10 km from the ridge
axis. This suggests that a narrow magma chamber is present beneath the ridge
axis at this location. A schematic diagram of the crest of the East Pacific Rise at
21
◦
Nisshown in Fig. 9.25.
The ultra-fast East Pacific Rise (half-rate 7.5-8.1 cm yr
−
1
) south of the Garrett
transform fault (13
◦
30
S) has also been the subject of seismic-reflection and
-refraction experiments. A high-amplitude, reverse-polarity reflector 0.5 s below
the seabed reflector is continuous over tens of kilometres along the ridge axis.
This reflector, which is only about 1 km wide, coincides with the top of a crustal
low-velocity zone, and appears to be from a narrow melt sill. The reflector is very
similar in width and thickness to those imaged on other ridges but is shallower
